This invention relates to process for manufacturing Calcite, calcium carbonate single crystal widely utilized as material for optical polarizers etc.
Single crystal of CaCO.sub.3 calcium carbonate for optical use is known as Calcite. When rays of light are made incident upon Calcite, it is characteristic that two refracted rays are produced from each ray incident upon calcite due to double refraction phenomenon. Particularly, because its compound refraction index is high, calcite is utilizted as polarizing prism for optical apparatus. Particularly because of the recent advancement of such apparatus as laser optics and optical communication etc. utilizing optics and because of increase in marketability, material with excellent optical characteristics is demanded. In this regard, Calcite is an ideal material, which is expected to be more and more in demand.
As to calcite, it is obtainable only from natural product because it is not yet industrially and artificially synthesized. It is required that natural calcite for the above is colourless and transparent, has no bubble and no crack inside, no twin, no strain inside, and a certain degree of size. However, Calcite that meets this requirement, is produced only in such limited areas as Republic of South Africa, Mexico, etc.
Trials to synthesize calcium carbonate single crystal have so far been made in a variety of ways. Namely, they include crystallization from solvent, synthesizing from gel, crystallization from flux or melt, or hydrothermal synthesis method etc, and recent FZ method or floating-zone technique under high pressure as well. However, optical characteristics such as transparencies has not been entirely satisfactory to be industrialized due to defects such as impurities, mixing or dislocation of inclusion, or strain in crystallization inside.
Among all these tried methods, the growth atmosphere of a hydrothermal synthesis method is the most similar to that of natural Calcite grown in the hydrothermal ore deposit. Therefore, there is a possibility to synthesize the crystal having characteristics closest to natural Calcite.
This hydrothermal synthesis process is a method utilizing aqueous solution at a fixed temperature and pressure in an autoclave in order to synthesize material, and is used for quality production of artificial crystal. Alkaline aqueous solution such as NaOH or carbonate aqueous solution such as Na.sub.2 CO.sub.3, K.sub.2 CO.sub.3 etc. is generally used as aqueous solution. As growth solution of calcium carbonate single crystal by hydrothermal synthesis process, carbonate aqueous solution has been tried as an extension of the above stated growth technology for artificial crystal.
Solvent--6 mol K.sub.2 CO.sub.3 aqueous solution
Temperature--410.degree. C.-445.degree. C.
Pressure--1720 atmospheric pressure
Growth speed--50 .mu.m/day
Under the above conditions, about 3 mm growth layer has been obtained.
The above described hydrothermal synthesis in the traditional manner is disclosed in the following literature: D. R. Kinlock H, R. F. Belt, R. C. Puttbac H, Journal of crystal growth 24/25 (1964) 610-613
In crystal growth method of calcium carbonate utilizing the conventional carbonate aqueous solution, crystal can be grown, but there are problems to be solved as follows:
Firstly, due to the high concentration of used solvent, there frequently occurs much inclusion in crystal. This will be a cause to obstruct optical characteristics.
Next, due to the high concentration of solvent it is impossible to control pressure by adjusting filling-up rate of solution at time of quantity-production of general artificial crystal. Namely, the higher the solvent concentration becomes, the lower the obtainable pressure becomes even with the same filling-up rate. In case of use of 6 mol concentration of K.sub.2 CO.sub.3 aqueous solution at 445.degree., and even if the filling-up rate is made nearly 100 pct, it is unable to obtain 1720 atm. (atmospheric pressure). Due to this, it is necessary to apply an exclusive pressure addition device to the outside of the autoclave, causing the device and pressure systems etc. too complicated. Further, in case that carbonate aqueous solution is utilized, the growth speed is very slow, i.e., 50 .mu.m/day. Therefore, it takes on the order of 1 year to grow crystal large enough to be used as optical elements. It is an object of the invention to provide a simple method to grow excellent calcium carbonate single crystal with desired characteristics of optical nature through a short period.